1. Field of the Invention
This invention relates to dc/ac inverters, and more particularly, to the constant-frequency, sinusoidal dc/ac inverters that utilize a coupled inductor to achieve zero-voltage switching of the switches in a wide range of input-voltage and load-current conditions.
2. Description of the Prior Art
Generally, high-frequency inverters employ a resonant or a soft-switching technique to reduce switching losses and, consequently, improve the conversion efficiency. However, the majority of these resonant- and soft-switched-type inverters require variable-frequency control to maintain a regulation of the output. While in many applications variable-frequency control exhibits an acceptable performance, a number of applications require a constant-frequency control. One of the most notable application that requires a constant-frequency inverter is the ac-distribution power system. In such a system, a constant-frequency sinusoidal or trapezoidal ac voltage is distributed to the loads for a final point-of-load conversion by load converters.
Constant-frequency control of inverters is implemented by the phase control, which is also called the "outphasing modulation." In this type of control, the output regulation is achieved at a constant frequency by phase shifting the switching instances of the corresponding switches in the two legs of the inverter. With no phase shift, the output delivers full power, whereas for a phase shift of 180.degree., the output power is reduced to zero. An extensive analysis of the series-resonant inverter (SRI) with phase control is presented in [1]. The same analysis can be extended to any other inverter topology such, for example, the parallel-resonant inverter (PRI), or the series-parallel resonant inverter (SPRI). Generally, the PRI and SPRI topologies are employed in applications which require no load operation since the SRI topology cannot regulate the output at no load because without a load the series resonant tank circuit is open.
As an example of prior art, FIG. 1 shows the circuit diagram of an isolated full-bridge PRI along with the timing of its control signals [2]. The parallel resonant circuit in FIG. 1 is implemented with primary-side resonant inductor L.sub.R and secondary-side resonant capacitor C.sub.R. However, it should be noted that the resonant capacitor could also be placed across the primary winding. Regardless of the placement of the resonant capacitor, the circuit in FIG. 1 offers a sinusoidal output voltage with a relatively low harmonic distortion, as well as the output voltage regulation in the entire load range from the full load down to no load. When the switching frequency of the circuit in FIG. 1 is below the resonant frequency, the primary switches turn off at zero current, whereas above the resonant frequency the primary switches turn on at zero voltage. The major deficiency of the PRI is a relatively low partial-load efficiency due to a significant circulating energy in the resonant tank, which is required to maintain soft-switching at lighter loads. As a result, the PRI is not suitable for applications that require low power loss at light loads as, for example, power supplies for personal computers.
The light-load performance of the PRI can be improved by employing the SPRI topology shown in FIG. 2 [3]. In the symmetrical SPRI circuit in FIG. 2, which consists of two bridge legs and multiple resonant components, inductors L.sub.R1 and L.sub.R2 form series resonant circuits with corresponding series capacitors C.sub.S1 and C.sub.S2 and parallel resonant circuits with capacitor C.sub.P. Because a properly designed SPRI circuit behaves like a PRI circuit at light loads and like a SRI circuit at higher loads, the SPRI can regulate the output down to no load with an improved partial-load efficiency. Nevertheless, although the circulating energy in the SPRI is reduced compared to that of the PRI, the SPRI still circulates an unnecessarily high energy to maintain soft switching in the entire load and input-voltage range. Moreover, because the Q-factor of a properly designed SPRI that minimizes the circulating energy is usually lower than the corresponding Q-factor of the PRI, the output voltage of the SPRI typically exhibits higher harmonic distortions than the output voltage of the PRI.